Traditionally, control of power transformers entails use of a mechanical thermometer with a high torque mechanism that can operate up to six switches to control cooling apparatus of the power transformer, such as circulating oil pumps and fans, with the switches being sellable by the user. The switches set in the highest part of their possible range are typically used for high temperature alarms and emergency shut down.
The two major types of power transformers are oil temperature controls and winding temperature controls. A winding control sensor is normally mounted in a thermowell that includes a heater having a current that is proportional to the main transformer load current. With the advent of computers, it has become popular to generate an electronic signal from both the oil and the winding control sensor for input to a computer for remote monitoring and to collect the temperature data history.
Thermometers with switches as output to control equipment have been widely used. For example, U.S. Pat. No. 3,114,903 (Murphy) includes a switch inside a case of a pressure gauge or thermometer for use as an alarm or safety shutdown.
Attempts have been made to replace the mechanical controls with electronic controls that offer advantages such as calculating and compensating functions. In this regard, U.S. Pat. No. 4,745,571 (Foster) describes a conventional mechanical system for control and monitoring of power transformers using separate winding and oil mechanical systems and improves on this conventional system by incorporating fully electronic controls. While some of these electronic controls are in use, the resistance of users to depend on electronics is strong in view of the harsh environments that electronics are subject to including for example, temperature extremes, both high and low humidity levels, high electromagnetic fields, and lightning. It is more common to combine the electronics as an output feature into the traditional filled system remote mechanical controls.
One common method for combining an output for computer monitoring is described by Messko in a technical brochure IN2100/00/01 entitled “Universal Retrofit Kit”. In the Messko control technique, a thin film pressure transducer is soldered into the liquid-filled thermal system within the case housing, and a correlation is made between pressure and temperature and converted to a 4 mA to 20 mA current output corresponding to 0° C. to 160° C. The current is typically connected to a precision resistor and the voltage across the resistor is read by an Analog to digital converter for use in a data acquisition computer, this 4 to 20 milliamp current loop being a common technique used in industrial automation and control.
Another method of integrating the output signal with a mechanical control is to use a precision potentiometer connected to the shaft of the mechanical control pointer in combination with a circuit to convert to a current loop signal. Both of these methods have the limitation of being dependent on the integrity of the filled systems, liquid filled in both cases, and having accuracy, repeatability and environmental errors no better than the filled system that they are dependent upon. Another disadvantage of these methods is that the entire control system would need to be replaced in the case of a failure requiring the transformer to be shut down.
The prior art systems inherently include errors due to ambient temperature changes because the liquid in the capillary and head usually expands with higher exposure temperature and contracts with lower exposure temperatures. A compensator is normally included in the instrument housing to offset this error at one point, usually at the mid point of the span, but no attempt is made to compensate for the change of the capillary temperature. The compensation techniques may include their own accuracy offset and need to be tested, sorted and matched to the thermal system and only at one temperature point. In addition, the compensation is not integrated in the electronic output thereby causing wide discrepancies between the mechanical and electronic systems, which in turn, cause an ambiguity regarding what the actual temperature is and thus whether to rely on the temperature measurement provided by the mechanical system indicator or by the electronic output.